The perceptual colour space of the scene
From the 'trichromatic' representation of the spectrum given by the three cone photoreceptors, the human visual system seems to apply a further transformation into three perceptual dimensions. One such dimension is achromatic and describes the apparent 'brightness'. The other two are called 'opponent colour' dimensions, because low values on the dimension correspond to one colour and high values on the dimension correspond to another colour. These dimensions are 'blue-yellow' and 'red-green'.
Note that we won't be going into precisely how this transformation is performed—it gets pretty complicated.
As before, we can visualise these dimensions for each pixel in our scene.
Discriminability estimates
We can estimate the discriminability of any two regions of the image by their distance in this three-dimensional perceptual space.
Click on a pixel in the scene image above, and the image below will show how perceptually discriminable that pixel is from the rest of the scene (with increasing brightness corresponding to increased discriminability).
So far, we have been assuming typical human colour vision. Next, we will explore some consequences of photoreceptor anomalies.